1. Field of the Invention
This invention relates to BGA (Ball Grid Array) semiconductor packaging technology, and more particularly, to a substrate strip which is composed of a series of BGA substrates and which is characterized by the provision of a warpage-preventive linkage structure that can prevent each substrate on the substrate strip from thermally-stressed warpage during high-temperature fabrication steps.
2. Description of Related Art
BGA (Ball Grid Array) is an advanced type of semiconductor packaging technology which is characterized in the use of a substrate whose front side is used for the mounting of a semiconductor chip thereon, and whose back side is implanted with a grid array of solder balls. During a SMT (Surface Mount Technology) process, the BGA package can be mechanically bonded and electrically coupled to an external printed circuit board (PCB) by means of these solder balls.
Conventionally, BGA packages are fabricated in batch on a substrate strip composed of a series of substrates. One problem of the conventional substrate strip structure, however, is that each substrate thereon would easily suffer from thermally-stressed warpage during high-temperature fabrication steps, such as during die-bond cure, wire bonding, molding, and molding cure, during which the temperature is typically about 200° C. The warped substrate would then cause an uncoplanarity problem to the subsequently implanted solder balls on the back side thereof, which would adversely affect the quality of the subsequent mounting of the BGA packages on external printed circuit boards (PCB). This thermally-stressed warpage problem is illustratively depicted in the following with reference to FIGS. 1A–1D.
FIG. 1A is a schematic diagram showing a sectional view of a typical BGA package. As shown, the BGA package is constructed on a substrate 11 whose front side 11a is mounted with a semiconductor chip 20 and whose back side 11b is implanted with a grid array of solder balls (i.e., ball grid array) 30. The semiconductor chip 20 can be electrically coupled to the substrate 11 by means of the well-known wire-bonding technology or flip-chip technology. During a subsequent SMT (Surface Mount Technology) process, the BGA package can be mechanically bonded and electrically coupled to an external printed circuit board (PCB) 40 by means of the ball grid array 30.
Referring further to FIG. 1B, in fact, BGA fabrication is typically implemented in batch on a substrate strip 10 which is composed of a series of substrates 11 supported on a frame 12 having a pair of parallel elongated supporting bars 12a, 12b. Each of the substrates 11 is used for the fabrication of an individual unit of a BGA package. Conventionally, each of the substrates 11 is rectangularly-shaped and linked to the supporting bars 12a, 12b by means of a four-point linkage structure consisting of four tie bars 13a, 13b, 13c, 13d on the four corners thereof. Typically, the upper-left tie bar 13a is also used to provide a gate (not shown) which is used for the injection of encapsulant during the fabrication of an encapsulation body (not shown) for encapsulating the semiconductor chip 20.
As shown in FIG. 1C, one drawback to the forgoing substrate strip 10 shown in FIG. 1B, however, is that during high-temperature fabrication steps, such as during die-bond cure, wire bonding, molding, and molding cure, each substrate 11 on the substrate strip 10 would be thermally expanded in all directions; but since the four corners of each substrate 11 are provided with the four tie bars 13a, 13b, 13c, 13d, the thermal expansion would be retarded in these directions; and consequently, the thermal stresses would concentrate toward the center of the substrate 11 (the directions of the thermal stresses are indicated by the arrows in FIG. 1C), thereby causing the center of the substrate 11 to be bulged out, resulting in a thermally-stressed warpage to each substrate 11.
As further shown in FIG. 1D, when the thermally-warped substrate 11 is implanted with the ball grid array 30, it would cause the implanted ball grid array 30 to have poor coplanarity. During the subsequent SMT process, this BGA uncoplanarity would cause some solder balls in the ball grid array 30 to be unreliably bonded to the PCB 40, thus resulting in a reliability problem in the BGA package.
The thermally-stressed warpage problem is particularly serious in large-size BGA substrates, including 35×35, 37.5×37.5, 40×40, and 42.5×42,5 (unit: millimeter) BGA substrates.
Related patents, include, for example, the U.S. Pat. No. 5,652,185 entitled “MAXIMIZED SUBSTRATE DESIGN FOR GRID ARRAY BASED ASSEMBLIES”, the U.S. Pat. No. 5,635,671 entitled “MOLD RUNNER REMOVAL FROM A SUBSTRATE-BASED PACKAGED ELECTRONIC DEVICE”; the U.S. Pat. No. 5,691,242 entitled “METHOD FOR MAKING AN ELECTRONIC COMPONENT HAVING AN ORGANIC SUBSTRATE; to name just a few.
The U.S. Pat. No. 5,652,185 discloses an inventive method of packaging a BGA assembly with a substrate that has been formed from a substrate strip whose area has been maximized. The U.S. Pat. No. 5,635,671 discloses a package assembly constructed on a substrate with a novel degating region to allow removal of excess encapsulant formed on the substrate surface during molding without damaging the remainder of the device. The U.S. Pat. No. 5,691,242 discloses an advanced method for packaging an integrated circuit on an organic substrate. All of these patents, however, utilize a substrate strip with the above-mentioned four-point linkage structure, so that the above-mentioned warpage problem during high-temperature fabrication stops still exists.